The conventional manual chain block, one of the manual hoist and traction machines, has been well-known as disclosed in, for example, the Japanese Utility Model Publication Gazette No. sho 54-39231.
The manual chain block disclosed in the above Gazette is so constructed that a driving shaft is supported between side plates through a pair of bearings, a load sheave in association with the driving shaft through a reduction gear mechanism is rotatably supported to the driving shaft, a driven member having at one side thereof a driven disc is coupled with the driving shaft, a boss of a handwheel is screwably mounted thereto, and a braking ratchet wheel engageable with a braking pawl and a pair of lining plates positioned sandwiching therebetween the braking ratchet wheel, are supported at the outer periphery of a cylindrical portion of the driven member, so that these members constitute a mechanical brake. A hand chain engaging with the handwheel is adapted to operate for rotatably driving the driving shaft through the mechanical brake and drive the load sheave to hoist a load or release the mechanical brake to lower the hoisted load.
The manual chain block constructed as the above-mentioned, however, uses a usual lining plate commercially available as a brake lining for a bicycle or the like for the lining plate to be used in the mechanical brake. Therefore, a diameter of the lining plate is made approximately equal to that of the handwheel at the handwheel and opposite to the side surface of the wheel.
The brake lining generally used for industrial machines, even when it has the most superior in properties, may have a coefficient of friction of 0.20 or more at a friction surface temperature of 200.degree. C. in consideration its function, and compressive strength may be 1000 (kg/cm.sup.2) at the largest and the maximum strain about 8 (10.sup.-3 mm/mm), which will comply with the provisions of an applicable Japanese Industrial Standard. When the usually commercial product is used for the mechanical brake of a manual chain block, in consideration of performance and quality, the lining plate is made larger in diameter, the friction surface of the wheel opposite to the lining plate increases in area and decreases in surface pressure, so that the mechanical brake is adapted to operate corresponding to a hoist load without hindrance.
Meanwhile, such a manual chain block has recently been required to be small-sized for the sake of simplicity in handling, which is realized by improving the quality of material of each component to raise its durability. However, the handwheel is limited in its diameter because of a limit to a diameter of the lining plate, thereby being restricted in its miniaturization. Also, the axial position of the handwheel needs to be set at the axially outside of the mechanical brake so that the axial size of the handwheel also is limited in the miniaturization thereof. When the handwheel is axially outwardly positioned, a distance between the mounting portions of the handwheel and bearing for supporting the driving shaft to the side plates, so that, when the handwheel is driven by the hand chain, the driving shaft is easy to deflect due to a load acting on the handwheel. Hence, in some cases where the hand-chain is forcibly operated, a problem is created in that the driving shaft is deformed. Also, in the no-load state where a hook at the load chain engaged with the load sheave hangs no load and the load sheave is not affected by the load, the body of chain block, as shown by the two-dot chain line in FIG. 7, is slanted due to weight of the hand chain engageable with the handwheel, whereby, when the hand chain 100 is operated to rotate the handwheel 101, not only the hand chain 100 comes into contact with a wheel cover 103 at the chain block body 102 so as to generate noises, but also resistance is generated against the operation of hand chain 100. Hence, there is no inertia resistance of handwheel 101 caused by the operation of hand chain 100, so that the hand chain 108 cannot quickly rotate the hand wheel 101, thereby creating the problem in that the hook of the load chain cannot be lifted rapidly.